Automobile parts and home electric product parts have often been press-formed from thin metal sheets, such as steel sheets and aluminum sheets, to a predetermined configuration using upper and lower tools having projections and indentations. Recently, materials of higher strength and reduced thickness have been employed in order to provide lightweight press-formed products. When such high-strength materials are press-formed, however, the materials may be elastically deformed and a product having a configuration different from a configuration at a bottom dead point of the tool may often be provided. When such springback (i.e., elastic recovery) is large in amount, part precision may decrease. In order to avoid decrease in part precision, the springback amount is usually reduced by modifying the product configuration to increase stiffness. The tool configuration is usually modified in advance expecting the springback amount.
Since press-formed products usually have complicated configurations, they are not uniformly distorted during forming. It is therefore not easy to specify springback-inducing areas. In order to address this problem, the following Patent Documents 1 to 3 disclose methods of specifying areas responsible for occurrence of springback. In the methods, the product is divided into plural areas on the basis of stress distribution at the bottom dead point of the tool in the product of interest, i.e., a target configuration of the product of interest. The springback is analyzed while stress in each area is changed sequentially. The methods disclosed in Patent Documents 1 to 3 use a finite element method described in Non-patent Document 1.
Patent Documents 4 to 7 disclose methods of controlling inner residual stress that may cause springback. In the disclosed methods, embossed portions and projections are previously formed at specified areas corresponding to characteristic product configurations, which embossed portions and projections will be flattened in the next step. Patent Document 8 discloses a method of forming embossed portions and projections in the entire surface of a raw material (i.e., a blank) which will be flattened in the next step.